1. Field of the Invention
The present invention relates to rare-earth-doped LiNbO.sub.3 laser devices and, more particularly, it relates to z-propagating and near z-propagating waveguide laser devices in rare-earth doped LiNbO.sub.3 in which the optical waveguide is oriented parallel or nearly parallel to the crystallographic z-axis of the LiNbO.sub.3.
2. Description of the Prior Art
The success of fiber amplifiers and lasers has recently stimulated a great deal of interest in rare-earth-doped planar waveguide devices for providing signal-processing functions on a local scale both in optical communications and sensor systems. In particular, rare-earth-doped LiNbO.sub.3 is extremely attractive for signal processing functions since the rare-earth-doped LiNbO.sub.3 potentially permits a high degree of integration through a combination of the existing mature waveguide fabrication techniques, the intrinsically good material properties in the rare-earth-doped LiNbO.sub.3, and the optical gain produced by the rare-earth ion dopants. Moreover, the incorporation of rare-earth ions in the LiNbO.sub.3 crystals by indiffusion demonstrates a degree of versatility not readily available in bulk rare-earth-doped planar waveguide devices.
Numerous integrated laser and amplifier devices have been demonstrated in the past in Nd- and Er-diffused LiNbO.sub.3. See J. Amin et al, Opt. Lett. 19, 1541 (1994); H. Suche, Proceedings of the 7th European Conference on Integrated Optics, session ThA4 (Delft, 1995), pg. 565. The most common method of waveguide fabrication in rare-earth-diffused LiNbO.sub.3 is by Ti-indiffusion allowing for low propagation losses and maintaining the spectral characteristics of the rare-earth ions. However, an inherent problem with Ti:LiNbO.sub.3 guided wave devices is the devices' relative instability at visible and near-infrared wavelengths as a result of photorefractive damage induced by the high optical power densities in these guides. This has limited the demonstration of cw room-temperature operation of Nd-doped devices almost exclusively to the case where the waveguides were fabricated by the annealed proton exchange process in MgO:LiNbO.sub.3.
Photorefractive damage has also been one of the main reasons that the majority of Er:Ti:LiNbO.sub.3 lasers and amplifiers have been pumped at 1480 nm. The only report of a 980 nm pumped Er:Ti:LiNbO.sub.3 device is described by Huang et al, Electron. Lett 32,215, 1996. It should be noted that the device described by Huang was only an amplifier. His work demonstrated no laser action. In the Huang et al reference, the detrimental effect of photorefractive damage on the amplifier gain was evident and it is unclear as to whether net gain was obtained in the device. It is widely accepted, however, that the photorefractive effect is due to photogeneration of electrons through ionization of Fe.sup.2+ impurities in the Fe.sup.3+ state, and the subsequent migration of these electrons along the z-axis (photovoltaic effect). As described by Becker et al, Appl. Phys. Lett. 47, 1024, 1995, trapping of the electrons, presumably in areas outside the waveguide, results in regions of space charge which perturb the waveguide modes through the electro-optic effect. In general, waveguides are fabricated in LiNbO.sub.3 with the propagation direction primarily perpendicular to the crystalline z-axis, in order to use the highest electro-optic coefficient (r.sub.33) for on-chip modulation. However, the space charge separation caused by the photovoltaic effect is on the order of the mode diameter, and therefore the associated fields remain largely within the waveguide, causing serious perturbation to the guided modes.
As was first reported by Holman, Proc. SPIE 408, 14, 1983, one way of considerably reducing the optical damage is by orienting the waveguide such that light is constrained to propagate substantially parallel to the crystalline z-axis. In this way, the charge separation is then along the guide length, and therefore the overlap between the fields associated with this separation and the optical mode is minimized. A disadvantage for the Holman z-propagation scheme is that it only allows for convenient use of the r.sub.22 electro-optic coefficient, which is lower than the commonly used r.sub.33 coefficient by a factor of approximately 9. However, the voltage requirement for switching in a z-propagating waveguide structure can be optimally made to be less than 15 V. Moreover, the effect of temperature changes in this z-propagating waveguide orientation, where both TE and TM modes are ordinary modes, are likely to be less than other orientations as dictated by the temperature-dependent Sellmeier dispersion equations and the associated temperature-dependent birefringence of the material. Also, because the z-propagating waveguide of the Holman reference does not support extraordinary modes, measures do not have to be taken during fabrication to suppress outdiffusion of lithium and spurious extraordinary waveguide modes which are known to arise from such lithium outdiffusion will not occur in the present invention. The fabrication of the present invention is therefore simpler. Even through the work of Holman illustrates the advantage of reduced photorefractive instabilities in optical waveguides which are oriented parallel to the crystallographic z-axis in LiNbO.sub.3, published work also exists which illustrates that in some instances the photorefractive damage may be significant. For example, in the paper of Sanford and Robinson, Proceedings of the 6th IEEE International Symposium on Applications of Ferroelectrics, 4 (1986), the authors show data which clearly indicates that the z-propagating waveguide geometry in LiNbO.sub.3 may exhibit serious polarization switching photorefractive instabilities. Furthermore, the same authors in a second paper, Proceeding of the SPIE, Vol. 704, 58 (1987), showed that these polarization switching artifacts may occur on the time scale of milliseconds. These polarization switching photorefractive artifacts were found in some cases to be so severe that upwards of 100% of the optical power could be exchanged between TE and TM modes. Consequently, with the work of Holman in conjunction with the work of Suche, in combination with the work of Sanford, a person skilled in the art would conclude that the z-propagating geometry is by no means an a-priori guaranteed success. Only demonstration of the fact that such a laser will indeed function, as done by the inventors of the present invention, and reducing the device to practice, as described herein, is conclusive evidence that such a laser can indeed by realized.
Thus, there is a need for a rare-earth-doped waveguide laser with improved stability at visible and near-infrared wavelengths. There is also a need for a rare-earth-doped waveguide laser with reduced photorefractive damage induced by high power densities. There is still a further need for an Er-doped waveguide laser and amplifier which can effectively be pumped at .gamma..sub.p =980 nm given that 980 nm pumping has proven to be more effective than 1480 nm pumping for locally pumped fiber amplifiers and the cost/mW of 980 nm pump diodes is currently lower than that of the 1480 nm diodes.